The present invention relates to a hearing aid defined in the preamble of claim 1.
Hearing aids comprise an acoustic input aperture to receive ambient sounds and an acoustic output aperture to emit the ambient sounds that were processed processed in the hearing aid. It is of foremost importance that spurious acoustic signalsxe2x80x94which subsequently would be processed as being ambient soundsxe2x80x94should not be superposed on these ambient sounds. Spurious acoustic signals may arise from airflows detaching off edges or in the vicinity of the hearing aid (detachment problems) or from airflows of different speeds and/or directions in the immediate vicinity of the microphone membrane which would cause this membrane to move (gradient problems). These two sources of spurious acoustic signals also may be encountered in combination.
The U.S. Pat. No. 4,073,366 describes a cover element which is bonded across the acoustic input aperture of a hearing aid. This known cover element of several layers bonded together at their edges consists of a porous material, its purpose being to preclude spurious acoustic signals generated by airflows detaching off the edges of the said input aperture.
It was found however that this known design only little affects the majority of the spurious acoustic signals: even though the cover element across the acoustic input aperture does reduce gradient-induced spurious acoustic signals, it fails to affect detachment noises. The reason is that the cover element mounted across the acoustic input aperture to reduce flow detachment by its own edges gives rise to likely new sources of spurious acoustic signals. In this design the source of interfering spurious acoustic signals only has been shifted.
Reference is made for the sake of completeness to the European patent document 0,310,866 which discloses covering the acoustic output aperture with a preferably micro-porous cover element to prevent ear wax from penetrating the hearing aid. Said document also discloses covering the acoustic input aperture with a cover element in case an in-ear hearing aid is involved because in such a case ear wax only might penetrate the hearing aid when latter is inside the ear. Steps minimizing noise interference cannot be inferred from this document which merely concerns the prevent of hearing-aid soiling.
Accordingly it is the objective of the present invention to create a hearing aid reducing the generation of spurious acoustic signal.
This problem is solved by the features of claim 1. The further claims address advantageous embodiment modes of the invention.
The invention offers the following advantages: Edges that would be sources of spurious acoustic signals are avoided by integrating at least one cover element into the hearing aid proper and in this manner the detachment problem has been met. At the same time the gradient problem also is solved in that the cover element is made of an open-pore material and assumes a given thickness.
It was found that an open-pore polyethylene is especially well suited as the cover element material. The material properties may be characterized on one hand by the filter fineness corresponding to the minimum particle size of the filtered particles and on the other hand by the open-pore ratio of the material, i.e. the ratio of pore apertures to residual surface. The filter fineness is stated in d50 values approximately, at which 50% of the particles pass the filter and 50% of them are retained by it. It was found that the d50 values are between 10 and 200xcexc as regards filter fineness and the open-pore ratio is between 0.70 and nearly 1.00.
Because a homogeneous material is used for the cover elements, they are also highly reproducible because material discontinuities, which might degrade the acoustic properties of the overall system, are absent, since changes in material hold the danger of deviations from the normal acoustic behavior. Moreover both the manufacture and the installation of the cover elements of the invention into hearing aids is substantially simpler and hence also more economical.
In an embodiment variation of the invention, the homogeneously constituted cover elements are coated with a thin and permeable layer for instance of Teflon. Higher resistance is achieved thereby and is highly significant especially as regards external chemical factors. However the detachment problem also is taken into account because the coating imparts a finer surface to the cover element. In this manner spurious, acoustic detachment signals generated by roughnesses in the cover element surface are further minimized.